clunking noise under load???

[ellisjay2]

Im getting a clunking noise under a load at driving speeds that comes and goes. When it comes I ease off accelerator or ease it on down to increase speed, clunk,clunk clunk, clunk,clunk,clunk,clunking noise gets louder, faster then goes away and im able to hit high driving speeds. I think i hear it through intake manifold as if it could be valves, or something thats rotating, possible lifter or push rod maybe?? It feels as if its a misfire on a cylinder at a certain point the way it stumbles and lacks power the few seconds when it happens, Im just trying to get an idea on where to aim though I know I will be rebuilding top end from what it sounds like soon because lacking power is no way to ride. Whats the best way to chase down a misfire, go through all the wires while running and pull loose to see how engine responds?? Or does deeper inspection need to occur, im all ears........

I will video when it makes clunking noise at idle while reving sounds as if its backfiring....

[Cabnchassis]

Tell us what year truck, what motor, what trans, etc. We can be more helpful if we know all that cause a lot of us only mess with certain motor/trans configurations.

[ellisjay2]

86 2500 Sierra classic, Carb 5.7, Th350 Rwd

[Bigstevex4]

Noises are hard to track down sometime describing them is critical to tracking them. What i think is a clunking sound if it was comming from an eng. Its not long for this worls tapping clicking yes clunking?

[Cabnchassis]

Ok so I don't mess with carbs very much so I can't help if your problem is there but a misfire is a misfire. Easiest way I know is to pull them one at a time, ground the plug on the block and look for spark.

As far as the top end goes, are you losing/using much oil? Does the truck smoke when you start it up? if so what color? Blue smoke usually means the valves or valves seats are letting oil drain into the cylinders and it burns off on start up.

Are you getting oil blow by? Blow by is usually a good wear indicator or a diagnostic tool

do you have any leaks on your valve covers? If so dried oil sludge could be impeding the function of your push rods, assuming the rods are close to the leak.

You might also want to check your dist gears for excessive wear since your dizzy spins constantly it could be a symptom of it's condition you're hearing.

[Cabnchassis]

Originally Posted by Bigstevex4

Noises are hard to track down sometime describing them is critical to tracking them. What i think is a clunking sound if it was comming from an eng. Its not long for this worls tapping clicking yes clunking?

Very true. The last time I was around a clunky engine, I ended up having to help my uncle drain the pistons out of the oil pan of his nova .

[ellisjay2]

Engine doesnt smoke, or burn oil, only smokes when cold start choke mixture not while driving. It is some oil debris around valve covers that I clean off with degreaser now and then, light but not heavy, Im not having to add extra amounts of oil often, it usually holds its regular amount, I may add a quart through the month from day to day driving. First chance will try to video/ picture of valve cover areas. It hasnt done it in last couple days, figured it would this mourning on way to work with cold weather at 35 degrees, rode just fine with passing speeds.

[Cabnchassis]

I know this is probably a stupid question but you know what I mean when i say a valve tapping noise? Like the way a 4cyl yota sounds no matter how much oil is in it. Is it that kind of click tap or are we sure it's a clunk? No trying to demean your mechanical knowledge, if that was inane, just making sure we cover everything before you start spending $$$ so clunking aside do you have any valve tapping?

Have you ever changed the egr valve? It is a common culprit in power loss and idle issues on any engine that has one. Thats as far as I could help you on smog for that engine things like MAP sensors and Oxy sensors can cause idle troubles.

I wouldn't look to valve issues first if you aren't burning a lot of oil, or hearing a lot of valve/valve seat chatter in there. You probably have enough leaks to account for that quart you're adding, my detroit leaks oil at the oil pan and probably the rear main seal but its hard to tell with the pan pooping oil out everywhere. It's had almost a whole quart some weeks My 350 knows the same trick, didn't even have to teach it. Pretty sure it only leaks at the oil pan. It leaks about a quart ever couple weeks if Im driving it a lot.

Check the oil dipstick for a burned mark closer to the top of the stick than the bottom. "Blow by" is combustion gas leaking around the piston rings into the crankcase. The vent for those gasses aims right at the dipstick in most vehicles i have experience with. When you are getting too much blow by that little scorch mark will be dark black or have that heated metal multi color look. Every engine has blow by because they are trying to contain gasses with a mechanical ring that rides in a steel cylinder, but you shouldn't really have too much in a gasser. Normal blow by levels in a diesel are heading for critical failure range on a gasser because of the insane compression ratio vs gas engines. Neat fact most non mechanics don't know, diesel oil turns black right away because that's what color the combustion gas blowing by the rings is. This probably isn't causing your problem but can be a good diagnostic tool.

If you have any oil exiting the valve covers there is most likely a deposit of dried up crusty oil building at the inside of those leaks. I had two small valve cover leaks in my k2500. One had minor build-up it was no big deal. The other leak would have stopped the push rod from functioning had I waited to change em.



The push rod in the center of the picture should have a cone shaped valley all the way around it but it is entirely full of sludge. The rod actually wore it's own way through the gunk pretty well but there was a golf ball size chunk of sludge building all around that valve cover leak on the inside. The stuff dries like concrete too and having the covers off it would be all too easy to dislodge the gunk farther into the engine which would be a big no no. In this particular case it is not affecting the operation of the rod and I stopped it from continuing, so I'm confident that it'll still be stuck right there still when this motor is dead, but I could have come out that deal a whole lot worse and had no clue it would be anywhere near that bad.

Another thing to consider and I know it's a long ways away from where you're hearing it but some of these trucks can get some really nasty drive-line vibration or imbalance if the U joints and Carrier bearing are old/bad.

Is the engine cutting out or stumbling on the norm? Do you have any major vacuum leaks? Wouldn't cause clunking (well I guess it could if something that articulated due to vacuum wasn't properly supplied?) but would cause power loss and stumbling. Sorry I don't know the vacuum system on the carbed 350s it's different that my TBI and thats the only GM gasser fuel system I've worked on. Use a can of diesel starting fluid if you aren't sure and with the truck running spray it around the carb base and around the intake and intake gasket or anything else that looks like it might be vacuum. If you hear the truck rev up you found a vacuum leak. On my 350 the vacuum leak was actually within the intake and there was another at that rubber hose for the fuel pump. I was ready to sell that truck over it but instead learned how the system functioned from guys on forums until I was able to diagnose my specific problems.

If you don't have A/C disregard this. I don't know if your set up would work this way but my 2500 constantly cycles the A/C compressor to dry the air in the cab. Every time it does that it clicks. Now I can't hear if my burb does that but I do know that for a while before the A/C pulley froze at 70 and slung the clutch all over the engine compartment, that pulley bearing or clutch was making a clunking noise for a while.

Lastly and it may not help at all take a video of it running if you can. Someone more experienced than us may have seen this issue and be able to hear it in an engine that we would think sounded normal.